1. Field of the Invention
This invention relates generally to displays and more particularly concerns an active matrix liquid crystal display cell in which smart spacers are provided having an anisotropic shape.
2. Description of Related Art
Active matrix liquid crystal displays (AM LCD) have a variety of uses including being used as the projection element in rear projection monitors. The AM LCD is generally constructed using a rear glass layer with an addressing element and pixel ITO electrodes, a liquid crystal layer, and a front glass layer with addressing elements and pixel ITO electrodes. The cell gap spacing between the front and rear glass layers should remain uniform for consistent light propagation through the AM LCD. Several conventional ways are known to assemble AM LCDs and achieve uniform cell gap spacing within desired tolerance levels.
FIG. 1 shows a cross-sectional view of a convention vacuum chuck or vacuum press method of assembling a liquid crystal display cell. In this method, a vacuum chuck 10 asserts a pressing force 24 upon a liquid crystal display cell. The liquid crystal display cell includes a bottom substrate 12, a top substrate 14, a seal 16 being an epoxy seal or other form of adhesive glue, liquid crystal material 18, and many randomly placed spacers 20. The spacers 20 are typically spherical glass beads or cylindrical micro-fibers and are randomly placed on the bottom substrate 12 using the techniques of the well known dry cloud method or the well known solvent dispersed method. Additionally, some spacers 20 are placed in the seal 16. The liquid crystal display cell is covered with saran wrap or flexible plastic sheet 22 and an ultraviolet light 26 cures the seal 16 while the pressing force 24 compresses the bottom substrate 12 and the top substrate 14 together.
FIG. 2 shows a conventional vacuum sealed plastic bag technique of assembling a liquid crystal display cell similar to the vacuum chuck or vacuum press method. The liquid crystal display cell includes a bottom substrate 12, a top substrate 14, a seal 16, liquid crystal material 18 and many randomly placed spacers 20. However, in this method, a vacuumed sealed plastic bag exerts the pressing force 24 on the liquid crystal display cell. The seal 16 is cured using either ultraviolet light or heat depending on the type of seal 16.
FIG. 3 shows a conventional balloon method of assembly a liquid crystal display cell. Again, the liquid crystal display cell includes a bottom substrate 12, a top substrate 14, a seal 16, liquid crystal material 18 and many randomly placed spacers 20. A balloon 30 and a thermal chuck hot plate 32 exert the pressing force 24 and cure the seal 16. The seal 16 is therefore thermally cured. On the other hand, if an ultraviolet light cured seal is used then the thermal chuck hot plate 32 may be replaced with glass and the liquid crystal display cell may be back irradiated with the ultraviolet light.
FIG. 4 shows a top view of a liquid crystal display cell assembled using any one of the vacuum chuck method, the vacuum sealed plastic bag method or the balloon method. A nine pixel arrangement is shown merely for illustrative purposes even though the actual number of pixels will be much greater. The liquid crystal display cell is divided into an active aperture area 34 and a non-active area 36 with the spacers 20 randomly distributed throughout both the active aperture area 34 and the non-active area 36.
The above-described conventional methods produce liquid crystal display cells with uniform cell thicknesses having acceptable tolerances. However, problems tend to occur as the size of the active aperture area 34 diminishes. For example, in some projection displays the size of the active aperture area 34 may be of the same order of magnitude as the spacers 20. When the spacers 20 overlap or rest on the active aperture area 34, they may occupy approximately 15% of the active aperture area 34, which significantly disrupts the performance of the liquid crystal display cell and degrades the resulting image. Additionally, the spacers 20 disturb the liquid crystal profile around the spacers 20 which further degrades the resulting image, (i.e., reduces the brightness and contrast).
The randomly placed spacers 20 may not be merely removed to leave the spacers 20 in the seal 16 as shown in FIG. 5 because the pressing force 24 results in a bend in the top substrate 14 that distorts the entire liquid crystal display cell and results in unacceptable image quality.
"Improved Construction of Liquid Crystal Cells" by Maltese et al., Alta Frequenza, Vol. XLVII, No. 9, pages 664-667, September 1978, the subject matter of which is incorporated herein by reference, discloses large multiplexed liquid crystal panels having distributed small-area spacers between plates to make a stiff structure.
U.S. patent application Ser. No. 08/767,652, filed Dec. 17, 1996, the subject mater of which is incorporated herein by reference, discloses at least one method of applying spacer elements randomly on a bottom substrate and subsequently removing the spacing elements from active areas.